The invention relates to a rod connection provided in a surgical device.
Surgical devices may be composed of several components, which are adjoined with each other depending on the application either upon assembly of the respective device by the manufacturer or at the instance of surgery. Thus a connection between those components has to be provided, which for example secures a predetermined degree of reliability of the device in view of strain, bending and torsion stress, when this connection represents a rigid or fixed interface between the components.
In clinical surgery, the materials selected for the distinct components of the surgical device may be, among others, bio-compatible metals such as Titanium or Nitinol, a Nickel-Titanium alloy, or synthetic materials such as PEEK (polyetheretherketone) optionally being carbon fibre reinforced. The type of connection between the components commonly also depends on the materials involved.
For example in the case of two components made from metal, a connection may be established by providing a bore in one component (rod receiving member), into which is press-fitted a rod-like member of the other component, which has an interference with regard to the bore. In other words, in a disassembled state, the rod-like member (rod part) has an outer diameter larger than the inner diameter of the bore.
The corresponding amount of interference is chosen depending on the materials involved, and more specifically depending on the respective friction coefficients. The interference is further chosen such that in case of both a maximum value and a minimum value of a tolerance range with respect to the corresponding diameters a reliable connection is maintained.
In various applications in clinical surgery there is a continuing demand to provide components having a reduced size while maintaining its reliability with respect to external force transfer, which also act on the connections established in the devices.
With regard to the above mentioned press-fit connections, it may be observed that with decreasing diameters of the bore and the rod, a minimum bilateral tolerance of 5 μm increases its influence on the connection design. Consequently, with decreasing sizes of the diameters, the relative amount of interference increases.
As a result, there is a tendency to provide rod connections in clinical surgery, which may require considerable axial forces to assemble the components.
Based on the foregoing, there is a need to provide a reliable rod connection which allows a rod to be inserted into a bore with a moderate axial force.